1. Field of the Invention
The present invention relates to a composite electrolyte membrane for a fuel cell. More specifically, the present invention relates to a composite electrolyte membrane for a fuel cell with good thermal stability and high ionic conductivity, a method for producing the composite electrolyte membrane, and a fuel cell including the composite electrolyte membrane.
2. Description of the Related Art
Fuel cells are energy conversion devices that directly convert chemical energy of fuel into electrical energy. Considerable research efforts have been made to develop fuel cells as next-generation energy sources on account of their high energy conversion efficiency and eco-friendly nature with low pollutant emission.
Particularly, polymer electrolyte membrane fuel cells (PEMFCs) operate at low temperatures, undergo no leakage by the use of solid electrolytes, and are driven at high speeds. Due to these advantages, PEMFCs have received a great deal of attention as power supplies in portable, automotive and household applications. In comparison with other types of fuel cells, PEMFCs generate high power with high current density, are simple in structure, and have fast start-up and response characteristics. PEMFCs can use methanol or natural gas as a fuel as well as hydrogen, and are highly durable. Moreover, PEMFCs can be reduced in size due to their high power density. For these reasons, PEMFCs are increasingly being investigated as portable fuel cells.
Dow, Nafion (DuPont), Flemion (Asahi Glass), and Aciplex (Asahi Kasei) are currently in use as solid polymer electrolyte membranes. Generally, such solid polymer electrolyte membranes use perfluorosulfonic acid polymer membranes that have a fluorinated alkylene groups in the main chain and terminal sulfonic acid groups in the fluorinated vinyl ether side chains (e.g., Nafion (DuPont)). Fluorinated polymer electrolyte membranes have good chemical stability and high hydrogen ion conductivity, but complicated fluorination processes entail considerable production costs, limiting their application to fuel cells for automobiles. Since fluorinated polymer electrolyte membranes have low water contents, it is necessary to operate cells at temperatures of 100° C. or higher in order to prevent catalyst poisoning. In this case, evaporation of water from the fluorinated polymer electrolyte membranes causes a marked reduction in ionic conductivity and stops the operation of the cells. Fluorinated polymer electrolyte membranes have low glass transition temperatures, which are responsible for their poor mechanical properties at high temperatures.
In attempts to overcome the above disadvantages, proton conducting polymer membranes have been developed in which a basic polymer, such as polybenzimidazole (PBI), poly(2,5-benzimidazole) or poly(2,6-benzimidazole), is doped with a strong acid, such as phosphoric acid. In a state in which no water is included, the acid imparts the proton conducting polymer membranes with conductivity by the Grotthus mechanism.
Polybenzimidazole and poly(2,5-benzimidazole) are less expensive than Nafion and have the ability to conduct protons at temperatures of 100° C. or higher without humidification. A trace amount of carbon monoxide inevitably remains in hydrogen produced from natural gas, gasoline or methanol. Carbon monoxide of a few ppm or more is adsorbed to the surface of platinum catalyst and impedes the oxidation of fuel at the surface, bringing about a marked reduction in fuel cell performance. When a fuel cell operates at a temperature of 120° C. or higher, platinum catalyst poisoning caused by carbon monoxide is greatly reduced because adsorption of carbon monoxide to the platinum catalyst is an exothermic reaction. In addition, the oxidation/reduction rate of the fuel cell can be increased, advantageously resulting in high cell efficiency.
For example, a reformer may be used in a fuel cell. In this case, heat may be released from the system. In the case where the fuel cell is designed to operate at a temperature of 100° C. or less, an additional cooling device is required to remove heat released from the system in order to prevent overheating of the system. In contrast, in the case where the fuel cell is designed to operate at a high temperature, such heat can be used for operation, thus avoiding the need for any additional cooling device.
Despite the above advantages, a fuel cell system using a basic polymer, such as PBI or ABPBI, has a fatal drawback in that an acid, for example, phosphoric acid, which is not permanently bonded to the basic polymer but is present merely as an electrolyte, may be dissolved from the polymer membrane due to the presence of water. Particularly, when a fuel cell operates at a temperature exceeding 100° C., a large portion of water as a reaction product formed at the cathode of the fuel cell escapes in the form of vapor through a gas diffusion electrode, causing less loss of phosphoric acid. However, a large quantity of water may be generated in some zones where the operating temperature is lower than 100° C. or the current density is high. Since the water is not immediately removed, it dissolves phosphoric acid, which shortens the cell life.
The mechanical properties of ABPBI deteriorate as the concentration of phosphoric acid increases. The polymer membrane is dissolved in phosphoric acid at a concentration of 80%. As a result, the power of the fuel cell markedly deteriorates with increasing number of cycles.
There is thus a need to develop electrolyte membranes that have good thermal and mechanical stability and can operate even at high temperatures.